Difference Between Enantiomers and Diastereomers

Main Difference – Enantiomers vs Diastereomers

Generally, isomers are molecules that have the same chemical formula but a different arrangement of its atoms. Isomers are mainly grouped into two broad categories: structural isomers and stereoisomers. Structural isomers have the same molecular formula but the atoms are connected in different ways. Stereoisomers are molecules that differ in the spatial arrangement of their structures. Stereoisomers are again divided into two groups known as enantiomers and diastereomers. The main difference between enantiomers and diastereomers is that enantiomers are found as mirror images whereas diastereomers are not.

Key Areas Covered

1. What is Chirality
      – Brief Explanation
2. What are Enantiomers
      – Definition, Structure, Properties
3. What are Diastereomers
      – Definition, Structure, Properties
4. What are the similarities between Enantiomers and Diastereomers
      – Common Features
5. What is the difference between Enantiomers and Diastereomers
      – Comparison of Key Differences

Key Terms: Stereoisomers, Chiral Center, Enantiomers, Diastereomers, Mirror Image, Isomers, Chirality

What is Chirality

Before going into detail about the difference between enantiomers and diastereomers, it is important to know what chirality is. For a molecule to have stereoisomers, the molecule should essentially be chiral. In order to have chirality, molecules must have four different groups attached to a central carbon. These groups should be chemically distinguishable. In these molecules, the carbon atom is called chiral carbon or chiral center. Some compounds may have multiple chiral centers.

What are Enantiomers

Enantiomers are organic molecules that are non-superimposable mirror images. This means that the spatial arrangement of one enantiomer molecule looks like a mirror image of the other enantiomer of it. But these two molecules are not the same; thus, they are not superimposable. Enantiomers have a central carbon atom which is called a chiral carbon and it is attached with four chemically distinguishable groups.  Since enantiomers are found in pairs, the two molecules are named as R-enantiomer or S-enantiomer. The letters R and S are given to name the chiral center of the enantiomer that gives the idea to which direction it is going to be rotated in the presence of plane-polarized light (R-clockwise rotation, S-counter clockwise rotation).

Enantiomers have identical physical properties except their ability to rotate plane-polarized light in equal amounts but to opposite directions. Enantiomers also have identical chemical properties except when another chiral carbon/chiral center is involved. Enantiomers always come in pairs. One enantiomer molecule can have only one mirror image, subsequently, one corresponding enantiomer molecule.

Figure 1: Thalidomide Enantiomers

What are Diastereomers

Diastereomers are a type stereoisomers that are not mirror images of one another. They are also non-superimposable due to the different spatial arrangement. Diastereomers can have more than one chiral center. Cis–trans isomers are diastereomers since they are are not mirror images and are non-superimposable.

Diastereomers have different melting points, boiling points, and densities. In other words, diastereomers have different physical properties and reactivity. Unlike enantiomers, diastereomers are not always recognized in pairs as they have no mirror images of each other. There can be several molecules which are diastereomers of each other.

Figure 2: Five Stereoisomers of Truxillic Acid which are Diastereomers of each other.

Similarities Between Enantiomers and Diastereomers

• Both enantiomers and diastereomers are stereoisomers.

• Both have chiral centers and are optically active.

• Enantiomers and diastereomers are non-superimposable.

Differences Between Enantiomers and Diastereomers

Definition

Enantiomers: Enantiomers are stereoisomers that are non-superimposable mirror images.

Diastereomers: Diastereomers are stereoisomers that are non-superimposable and are not mirror images.  

Number of Isomers

Enantiomers: Enantiomers are always in pairs.

Diastereomers: There can be several molecules which are diastereomers of each other.

Physical Properties

Enantiomers: Enantiomers have identical physical properties except the ability to rotate plane-polarized light.

Diastereomers: Diastereomers have distinct physical properties.

Morphology

Enantiomers: The shape of the molecule is similar in enantiomers.

Diastereomers: Diastereomers have different molecular shapes.

Conclusion

Isomers can be divided into two broad categories named as structural isomers and stereoisomers. Among them, stereoisomers are optical active due to their presence of chiral centers. Enantiomers and diastereomers are the two types of stereoisomers. The main difference between enantiomers and diastereomers is that enantiomers are found as mirror images but diastereomers are not mirror images.

References:

1. Brown, William H. “Acetic acid (CH3COOH).” Encyclopædia Britannica. Encyclopædia Britannica, inc., n.d. Web. Available here. 07 June 2017. 
2. Helmenstine Anne Marie. “Glacial Acetic Acid Versus Ordinary Acetic Acid.” ThoughtCo. N.p., n.d. Web. Available here. 07 June 2017. 

Image Courtesy:

1. “Acetic-acid” (Public Domain) via Commons Wikimedia
2. “AceticAcid012” (Public Domain) via Commons Wikimedia